Recent years have seen on-going development of optical read/write apparatus capable of writing a large amount of data, like video data in digital format, and randomly accessing such data. Also, various attempts are being made to increase the storage density of optical disks used as storage media in such optical read/write apparatus.
In optical read/write apparatus, attempts are being made to increase storage density by means of, for example, an increased numerical aperture of an objective lens and the use of short wavelength illumination for a smaller light beam spot. The efforts have been successful and the storage capacity optical disks are getting larger year after year. Technology has already established as to a DVD-ROM (Digital Versatile Discs for Read Only Memory) as an optical disk which now has doubled its capacity owning to double layer structure.
A document entitled “A 16.8 GB Double-Decker Phase Change Disc” distributed in Joint International Symposium on Optical Memory and Optical Data Storage 1999 discloses an optical disk with an added density thanks to the double data storage layers which are writeable and readable.
In the optical disk disclosed in the document, each data storage layer is made of phase change material. Such optical disks are classified into two types: Low-to-high media which has a higher reflectance in recording mark areas than in interval areas interposed between recording mark areas and high-to-low media which conversely has a higher reflectance in interval areas than in recording mark areas. Both types of media enable the readout of data by means of quantities of reflected and transmitted light which vary depending on whether the phase change material is in polycrystal or amorphous phase. Similar optical disks using phase change material are disclosed in, for example, Japanese Laid-open Patent Application 2001-52342 (Tokukai 2001-52342, published on Feb. 23, 2001).
However, for example, on the high-to-low medium having a higher reflectance in interval areas than in recording mark areas, mark rows which include low reflectance amorphous areas are formed along guiding grooves in recorded areas. In the optical disk, data is written or read on a first data storage layer close to the light-striking side and on a second data storage layer far from the light-striking side using light incident to the same side of the disk, the light beam first travels through the first data storage layer before writing or reading data on the second data storage layer. Accordingly, upon writing or reading on the second data storage layer, the intensity of light beam reaching the second data storage layer after passing through the first data storage layer must differ depending on whether or not the first data storage layer already holds any records, so as to produce different writing or reading power sensitivities with respect to the second data storage layer.
Therefore, to write or read data on the second data storage layer, the first data storage layer must be checked first to determine whether there are any records on it, so that the write or read light beam intensity can be specified. This adds complexity to the write/read system. A problems arises here that optical writing/reading system using such an optical disk is hardly practicable.
As mentioned above, Japanese Laid-open Patent Application 2001-52342 discloses an optical disk having a double data storage layer structure in which address information is provided in the form of wobbling groove so as to achieve stable writing and readout.
Referring to FIG. 64, an optical disk 501 provided with conventional double data storage layers has a center hole 502 at the center. Data is written/read in a recordable area 503 in which a spiral guiding groove is provided for data write and readout.
The optical disk 501 has an address area 504 occupying a certain angular part. Address information is stored in the address area 504 as address pit rows extending radially. Throughout this text, this configuration, in which address information is stored collectively in one place, i.e., the address area 504 in the case of the optical disk 501, will be referred to as a lumped address scheme.
FIG. 65 shows the optical disk 501 in vertical cross section. The optical disk substrate 506 has thereon a guiding-groove-formed layer 507 on whose surface a spiral guiding groove is formed from depressions and projections, a second storage layer 508, a guiding-groove-formed intermediate layer 509, a first storage layer 510, surface-coating layer 511 which are deposited in the order. To write/read data on the first storage layer 510 and the second storage layer 508 in the optical disk 501, a focused light beam 512 is shone onto the first and second storage layers 510, 508 via only one side of the disk, that is, the side of the surface-coating layer 511.
FIG. 66 shows an enlarged view of a guiding groove 513 and a part of address pit rows 515 in the address area 504. On the optical disk 501, recording marks 1114 are formed along the spiral guiding groove 513, and the address pit rows 515 are formed extending from the guiding groove 513 in the address area 504.
To read/write data on the first storage layer 510 in the optical disk 501, as shown in FIG. 67, the light beam 512 to focused to illuminate the first storage layer 510 by means of tracking along the guiding groove 513 on the first storage layer 510 while controlling the intensity of the light beam. To read/write data on the second storage layer 508, the light beam 512 is focused to illuminate the second storage layer 508 by means of tracking along the guiding groove 513 on the second storage layer 508 while controlling the intensity of the light beam.
Under these conditions, let us suppose that the optical disk 501 is a phase change storage medium of a high-to-low type in which, for example, interval areas have high reflectance, i.e., lower transmittance, than the recording marks 1114 on the first storage layer 510 and the second storage layer 508.
In the event, to read/write data on the second storage layer 508, a light beam 512d passes through the area where there is the guiding groove 513 on the first storage layer 510 and is focused onto the second storage layer 508, only after having passed through the area where there exist the recording marks 1114 which have relatively better transmittance. In contrast, a light beam 512d passes through the address area 504 of the first storage layer 510 and is focused onto the second storage layer 508, only after having passed through the area where there are no recording marks 1114 which have higher transmittance, that is, a low transmittance area. Therefore, the intensity of the light beam 512e having passed through the area where there is the guiding groove 513 on the first storage layer 510 becomes greater than that of the light beam 512d having passed through the address area of the first storage layer 510.
Therefore, referring back to FIG. 66, as to the optical disk 501 having address area where address pit rows 515 are lumped together, the intensity of a light beam focused onto the second storage layer 508 varies between the address area 504 and the other area where the guiding groove 513 is provided. This makes it impossible perform stable write/readout.
To solve these problems, in the aforementioned prior art patent publication, no address area 504 with address pit rows 515 in FIG. 66 is provided. Instead, it suggests that the variations in intensity of the light beam focused on the second storage layer 508 be restrained by providing a wobbling guiding groove to record address information in the form of wobbles. Throughout this text, the configuration, in which address information is not stored collectively in one place, but distributed will be referred to as a distributed address scheme.
However, in the configuration disclosed in the prior art patent publication, address information is stored on the guiding groove in the form of its wobbles. Therefore, the guiding groove needs be scanned over a relatively long period of time to retrieve a single set of address information.
Specifically, each address pit in the address pit rows 515 in FIG. 66 has a diameter which is more or less equal to the width of the guiding groove 513: typically, 0.3 microns to 0.5 microns, and each set of address information is recorded over about 1 mm or less of the guiding groove 513 in the address area 504.
In contrast, in the case of wobbling guiding grooves, to ensure that the quantity of reflected light does not vary in tracking, each wobble must be several tens of microns long, that is, each address area storing a set of address information must be about 100 mm long in a wobbling guiding groove.
In a lumped address scheme using address pit rows 515, address information is completely reproduced when about 1 mm or less of the address area is scanned. Meanwhile, in a distributed address scheme using a wobbling guiding groove, address information is completely reproduced only when about 100 mm of the guiding groove is scanned, which is relatively long. Distributed address scheme is therefore not to achieve high speed randomly access in optically reading/writing data on optical disks. Lumped address scheme should hence be employed to reproduce address information instantly.
Now referring to FIG. 68, another conventional optical disk 601 has a center hole 602, a recordable area 603, innermost part 604, an outermost part 605, and prepit areas 606.
The optical disk 601 is provided with a guiding groove (not shown) which is, for example, spiral. Tracking is done along the guiding groove to read/write data in the recordable areas 603 by shining a light beam 621 onto first and second storage layers (double layers) 611, 612 as shown in FIG. 69. In the prepit areas 606, or the inner prepit area 606a and outer prepit area 606b, of the first and second storage layer 611, 612, are there formed pit rows (not shown) which form, for example, a spiral. Tracking is done along the pit rows, and a light beam 621 is shone to reproduce prerecorded information from the pit rows.
FIG. 70 shows an enlarged view around the border between the recordable area 603 and a prepit area 606. FIG. 71 shows its cross section in which only the first storage layer 611 and the second storage layer 612 are depicted. The following description assumes that the first and second storage layers 611, 612 are formed in a phase change storage medium of a low-to-high type whose transmittance is higher in produced recording marks than in non-recorded areas.
As shown in FIG. 70 and FIG. 71, if the first storage layer 611, located on the light-striking side, has a prepit area 606, light beams 621a, 621b are focused and shone onto the second storage layer 612 after recording marks M are formed along the guiding groove G in the recordable area 603 of the first storage layer 611. In this case, intensity differs between the light beam 621a, which is transmitted through the recordable area 603 and then focused, and the light beam 621b, which is transmitted through the prepit area 606 and then focused.
In the recordable area 603 do there exist multiple recording marks M with high transmittance, and the light beam 621,a transmitted through the recordable area 603 of the first storage layer 611 has a relatively high intensity. In the prepit area 606 do there exist no recording marks M, and the light beam 621b transmitted through the prepit area 606 of the first storage layer 611 has a relatively low intensity. As could be understood from this, the provision of a prepit area 606 in the first storage layer 611 causes undesirable variations in reading/writing power in reading/writing and makes it impossible to read/write data on the second storage layer 612 in a stable manner.